The usual commercial method of separating undesired low-boiling species from silanolterminated diorganopolysiloxanes consists of ordinary batch vacuum distillation. Heat transfer is always a problem in such processes, causing sometimes localized overheating, color development, loss of product and the like. An improved batchwise method of stripping volatile species from silicone polymers is disclosed by Hatch et al in U.S. Pat. No. 2,834,754. However, the process calls for kneading the polymer mixture in an apparatus suitable for such a purpose, e.g., a Baker-Perkins or Banbury mixer, and as is well known, such machinery is heavy and expensive. Continuous vacuum stripping has also been proposed as a means for removing such volatiles from silanol-terminated diorganopolysiloxanes, but this requires higher temperatures than are desireable, and greater energy usage.
Now it has been discovered that the relatively lowboiling species, e.g., cyclic polysiloxanes, can be removed from such silanol-stopped diorganopolysiloxanes, continuously and under unexpectedly mild conditions, by a method which employs steam added to the product before the devolatilization step.
More particularly, if steam and the cyclic-containing silicone polymer are each fed to an evacuated tortuous, confined passageway, such as a parallel plate stripping unit, at a moderately elevated temperature and under moderate vacuum, the volatile content of the polymer can be efficiently reduced continuously and quickly from an initial amount of about 9 to 18% by weight of the feed, on a steam-free basis, to less than 2% by weight. This process is more advantageous than batch-wise methods because it is continuous. The process of the invention also has major advantages over continuous vacuum distillation methods which do not employ steam in that it accomplishes the desired stripping of volatile cyclics under mild conditions, with greater devolatilizing effectiveness, and without degradation of the polymer.